1. Field of the Invention
The present invention relates to a semiconductor laser. The semiconductor laser of the invention is applicable to all types of semiconductor lasers of which the oscillation wavelength changes owing to current/light output/temperature, etc.
2. Description of the Related Art
Remarkable progress has been made in recent technologies in optical information processing and optical communication. For example, to realize high-speed two-way communication by optical fiber networks for transmission of a large quantity of information such as image information, a large-quantity optical fiber transmission line and also a signal amplifier flexible to the transmission line are indispensable. As one typical example of the case, studies of an optical fiber amplifier doped with a rare earth element such as Er3+ (EDFA) are now made in various fields. With that, it is desired to develop an excellent semiconductor laser for an excitation light source, which is an indispensable element as a component of EDFA.
In principle, the oscillation wavelength of the excitation light source applicable to EDFA includes three, 800 nm, 980 nm and 1480 nm. Of those, it is known that the excitation at 980 nm is the most desirable in view of the properties of the amplifier and in consideration of gains and/or noise figures thereof. A laser having such an oscillation wavelength band of 980 nm has been realized by providing an active layer of InGaAs on a GaAS substrate, and it must satisfy two contradictory requirements that its output power is high and its life is long. Further, SHG light sources and others require the wavelength around it of, for example, from 890 nm to 1150 nm, and the development of lasers having excellent properties is desired in other various application fields.
In the field of information processing technology, short-wave semiconductor lasers are being developed for attaining high-density data storage. In particular, the recent development of blue lasers is remarkable, and the reliability of a GaN material grown on an AlOx or the like substrate is increasing, and further studies are now being made on these. Further, semiconductor lasers are applied to the field of medicine and also to the field of precision fabrication, and their application range will further increase in future.
In general, semiconductor lasers are small-sized and light-weight as compared with solid lasers and gas lasers, and they have many applications owing to such their advantages. However, semiconductor lasers are not always superior to any other laser light sources from the standpoint of the wavelength stability thereof. For example, in an ordinary Fabry-Perot semiconductor laser in which the facet reflection is the basis of the cavity constitution thereof, the oscillation wavelength generally increases with the increase in the device temperature. This is because the band gap of the material that constitutes the semiconductor laser reduces at high temperatures, and it may be said that the characteristic of the device is basically intrinsic to the constitutive material thereof. When the device is driven for high output power operation, or that is, when the input current to the device is increased, then the oscillation wavelength of the semiconductor laser may generally increase owing to the heat generation by the device. In general, it is desired that the wavelength fluctuation in semiconductor lasers depending on temperature/light output/input current changes is as small as possible, and it is desired to solve the problem.
Various attempts have been made for providing semiconductor lasers with small wavelength change. For example, as so described in H. C. Casey, Jr., M. B. Panish, Hetero-structure Lasers (Academic Press, 1978), pp. 90-106, it has been known that a semiconductor laser (DFB laser) with stabilized oscillation wavelength could be produced by forming a periodical grating structure, as built in around the active layer of the device, and using it as a distributional reflector. Another method of wavelength stabilization has been employed broadly, which comprises forming an external cavity structure for a semiconductor laser, and in which the light having a specific wavelength of the outputted light from the semiconductor laser is selectively reflected and it is inputted into the device. However, the former requires the formation of a periodic grating structure inside the device, and therefore has some drawbacks in that the process is complicated and the device is unsuitable to high output driving operation. The latter requires an external cavity structure and the light source is therefore large-sized as a whole, and its problem is that it loses the advantage of small-sized semiconductor laser body.
On the other hand, in the present inventor's report, IEEE Journal of Quantum Electronics, Vol. 36, No. 12, December (2000), pp. 1454-1461, a semiconductor laser at a 980 nm band is formed on a substrate transparent to a light having the oscillation wavelength thereof. Precisely, in this, when the refractive index of the substrate is relatively larger than that of the clad layer, or that is, when a substrate capable of expressing a waveguide function exists under a semiconductor laser waveguide that is intentionally formed in the device, and when the laser waveguide is coupled to the substrate waveguide, then (1) depending on the substrate thickness, the device oscillation spectrum includes intensity modulation irrespective of the Fabry-Perot mode spacing defined by the cavity length of the device (FIG. 4 in the reference); (2) the intensity modulation period is 2.5 nm or so when the substrate has an ordinary thickness (120 μm or so); and (3) in that situation, the current dependency/temperature dependency of the longitudinal-mode that shows the maximum intensity in the oscillation spectrum of the device exhibits step-like specific characteristics (FIG. 7 and FIG. 11 in the reference). As in FIG. 7, these characteristics include a region in which the current dependency of the oscillation wavelength is extremely small in an extremely minor region thereof, and this characteristic gives some suggestion for wavelength stabilization of semiconductor lasers.
In addition, in the reference, the wavelength stabilization mechanism is discussed. In general, the spectrum of the gain generated by current injection into the waveguide, that is built in as a semiconductor laser, moves toward the long wavelength side along with the increase in input current/light output/temperature. This is the reason of wavelength change in ordinary semiconductor lasers. Hereinafter, the phenomenon, which expresses wavelength shift toward the long-wavelength direction is referred to as “red-shift”. However, when a semiconductor laser is formed on a substrate transparent to a light having the oscillation wavelength and when the refractive index of the substrate is relatively larger than that of the clad layer, then a mechanism of inhibiting wavelength change is created inside the substrate. In the substrate that expresses the function as a waveguide, no stimulated emission occurs and therefore the inputted carrier is accumulated therein. In general, the refractive index of a semiconductor material decreases with the increase in the carrier density thereof. This phenomenon is known as a plasma effect. Accordingly, on the intensity modulation in the oscillation spectrum that is generated as a result of coupling of a laser waveguide and a substrate waveguide, and on the longitudinal-mode selected according to the result, the wavelength-shortening mechanism shall act through current injection. Hereinafter, the phenomenon, which expresses wavelength shift toward the short-wavelength direction is referred to as “blue-shift”. Specifically, it may be understood that the wavelength stabilized region seen in FIG. 7 in the above-mentioned reference, IEEE Journal is a result that is realized by the “balance” of the effect of the gain spectrum incidental to the laser waveguide of which the wavelength is red-shifted as a result of current injection, and the effect derived from the substrate waveguide of which the wavelength is shortened by the plasma effect. Regarding the temperature dependency, since the effect of red-shift of the oscillation wavelength owing to increase of the refractive index of the substrate due to temperature increase is smaller than the effect of red-shift of the oscillation wavelength owing to the reduction in the band gap of the active layer due to temperature increase, it is possible to inhibit the effect of the gain spectrum of the laser waveguide of which the wavelength is red-shifted as a result of the temperature increase.
However, in the above-mentioned system, the region in which the wavelength is stable to the current change is narrow, as in FIG. 7 of the above-mentioned IEEE Journal reference, and there is a problem in that an extremely large wavelength change occurs before and after the stabilized region. For solving it, however, it is difficult to control the waveguide mechanism of the substrate. This is because of the following reasons: The substrate plays a role as an underground for epitaxial crystal growth thereon in forming an LD structure, and, in addition to it, the substrate must be thick enough to ensure the mechanical strength thereof so as to protect wafers from being broken in a process of producing semiconductor lasers, and, on the contrary, it will have to be thin to such a degree that it could be cleaved to form semiconductor laser facets. Accordingly, the overall thickness of the device will have to be defined to finally fall between around 100 μm and 150 μm or so. Even when the optically-optimum thickness of the substrate is 40 μm, it is really impossible to handle the substrate of the type.